1. Field of the Invention
The present invention relates generally to furnaces, and more particularly to a vacuum thermal furnace used in a manufacturing process to anneal semiconductor devices.
2. Description of Related Art
Semiconductor device manufacturing has evolved into a delicate and sophisticated process requiring state of the art processing equipment, sophisticated clean room facilities and precise and accurate metrology equipment. Semiconductor devices are typically manufactured by successively depositing and patterning layer after layer of ultra-pure materials on a substrate. These layered materials are often as pure as one part per million and are often deposited with thicknesses as precise as a few Angstroms. The deposition process is usually performed according to by some well understood process such as physical vapor deposition (PVD), chemical vapor deposition (CVD), ion beam deposition (IBD), plating, etc. Semiconductor devices are built up by depositing these thin films onto substrates made of an elemental semiconductor (e.g., silicon or germanium) or compound (e.g., gallium-arsenide or indium-phosphide) semiconductor, patterning the films using photolithography and etching processes, and then repeating the same processes using different materials and patterns. In addition to thin film deposition and photolithography processes, there are other processing steps such as annealing, burning-in, and electrical testing, which are required to make semiconductor devices.
Modern processing techniques in most of these areas have been developed to manufacture semiconductor devices with dimensions so small that an instrument, such as an atomic force microscope, is required to observe the different details of millions of devices on the substrate. The degree of difficulty in manufacturing semiconductor devices has significantly increased over the years because of both this miniaturization and increased complexity of semiconductor devices.
For example, in modern manufacturing environments, allowable levels of contaminating particulates in process equipment and clean rooms are often less than one particle per cubic foot, compared to several hundred particles per cubic foot a few years ago. Process gas purity of five-nines (i.e., 99.999%) or better is now required to run most processes, as compared with three-nines (i.e., 99.9%) purity in the past. Further, thin film thicknesses are often controlled to within a few Angstroms across the entire substrate, as opposed to a few nanometers. Additionally, processing windows have become extremely narrow, requiring Cpk values (statistical process capability indices) of four sigma or better. In order to achieve these stringent manufacturing requirements, semiconductor-processing equipment has become more sophisticated. One process area, which has been forced to adapt to these new high levels of performance, is the annealing process. Requirements on temperature control and uniformity across the substrate, reduction in particulates on the substrate, reduction in gas contamination, and improved cleanliness has imposed more stringent requirements for furnace performance and design.
Annealing processes have a direct affect on the texture of a copper or other metal-film layer such as tantalum or molybdenum. The size and orientation of the grain of the layer are controlled by the annealing process, and are critical to the electrical performance of the metal-film layer. This is especially so in microcircuits with very small line-widths that include deep and narrow trenches.
In addition, advancements in the development of copper interconnects has brought about the inclusion of other metals such as bismuth and magnesium as composites within the copper thin film. This requires additional thermal annealing treatments or alloying for homogenizing the composites, electrically activating the composites, and to provide contacts to other layers within the microcircuit.
Annealing processes are typically performed by furnaces such as a Rapid Thermal Processor (RTP) 100 displayed in FIG. 1. Typically, the RTP 100 includes a process chamber 110, a substrate stage 120, a heating lamp assembly 130, a process chamber door 140 and a motor shaft assembly 150. The RTP 100 radiantly heats wafers or other substrates in a vacuum environment. Once a substrate is loaded into the RTP 100 process chamber 110, the process chamber 110 is evacuated with a vacuum pump (not shown) and the heating lamp assembly 130 is powered on. Disadvantageously, substrates heat up very quickly and temperature is difficult to control, especially at lower temperatures. Furthermore, temperature uniformity across a substrate is difficult to control because different lamps radiate with different intensities. In fact, non-uniform heating becomes a larger problem as lamps degrade and intensity differences between lamps increase. If a lamp burns out, the heating uniformity across the substrate becomes unacceptable for proper processing to occur.
In addition to non-uniform heating problems, conventional furnaces have proven to be inadequate for annealing newer materials used in semiconductor devices such as copper, titanium, and tantalum. Although new techniques for thin film deposition have been developed which make, for example, copper thin film deposition feasible, there has been little advancement in other processes, such as the annealing process. As a result, the transition from aluminum conductors to copper conductors has been very slow.
Since copper is more difficult to process than aluminum, there are many problems that must be overcome before copper conductors completely replace aluminum conductors. Some of the problems associated with using copper include difficulty in producing fine copper patterns found in integrated circuits, difficulty in polishing and planarizing a copper coated substrate, copper migration, and copper contamination. Annealing conditions directly contribute to all of these problems because copper is extremely sensitive to high temperatures, and as a result, needs to be annealed at a relatively low temperature (100° C. to 400° C.). These low temperature annealing conditions make the RTP 100 as well as other semiconductor furnaces unsuitable for processing advanced materials.
Although the RTP 100 or diffusion furnaces have conventionally been used to anneal copper, these furnaces are incapable of properly annealing the copper at a controlled low temperature. One disadvantage of conventional diffusion furnaces is that temperatures ramp up and down slowly, thus requiring an extended period of time for the annealing process. Another disadvantage of conventional diffusion furnaces is non-uniform heating of substrates from a substrate center to an outer edge (this non-uniform heating is referred to as “RTD” or Radial Thermal Delta). This non-uniform heating causes the copper on the outer edge of the substrate to anneal more quickly than the center (assuming a higher temperature on the edges relative to the center), creating poor uniformity across the substrate.
Although the RTP 100 heats rapidly, unlike the diffusion furnace, the RTP 100 also has disadvantages. The RTP 100 heats substrates rapidly to elevated temperatures by activating heat lamps, which have a tendency to heat very rapidly. These heat lamps are ineffective at low temperatures. Further, the RTP 100 lacks the capability of temperature control, which is required for proper copper annealing.
Hotplates may also be used for quick annealing associated with the chemical mechanical planarization (CMP) process used in copper electroplating. This type of annealing system fits into the mechanical schemes of the electroplating systems. A substrate is set on the hotplate where it is annealed. Certain hotplate systems isolate the substrate from the plate with a blanket of inert gas. Hotplates, however, can produce “hot spots” on the substrate, which can produce a non-uniform anneal that can cause hillock peaks that make the planarization process less effective.
Accordingly, there is a need for a vacuum thermal annealer. There is a more specific need for a thermal annealer with proper temperature control for use with high conductivity materials such as copper.